Question

How can I replace some words in a string with some other words? For example:

var text1 = "This is a sentence. It is a pencil."
text2 = modify(text1);

I want text2 to be “That was a sentence. I was a pencil.”

So modify function replaces This->That , is->was

Answer 1

To replace all instances of the substring is with was you can use the replace^[MDN] method:

text2 = text1.replace(/is/g, "was");

Note that because is is a part of the word this, it will actually return

Thwas was a sentence

If you wanted to replace all instances of This to That and is to was, you could chain the calls to the replace method.

text2 = text1.replace(/This/g, "That").replace(/is/g, "was");

This will correctly do your replacement from

This is a sentence. It is a pencil.

to

That was a sentence. It was a pencil.

You can see this in action on jsFiddle.

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Note that find and replace actions like this can always have unintended consequences. For example, this string

Thistles and thorns are bad for missiles and corns.

will turn into this one after your replacement:

Thatles and thorns are bad for mwassiles and corns.

This sort of thing is popularly known as the Clbuttic mistake.